Static inverters are very similar to UPS systems, except that they are usually configured to operate in "active standby" mode. This means that the bypass will supply the load during normal conditions. When a mains failure occurs, contactors are used to transfer to the inverter. The standby batteries will then support the load (typically for 1 hour or 3 hours depending on the application).
This type of product is perfect for emergency lighting applications, as it offers efficient backup and reduced running costs. It should be noted, however, that a static inverter does not provide any protection from power surges or voltage spikes. Although it doesn't offer power conditioning, it is a more affordable option than a traditional UPS system.